58 ZOOLOGY 



CHAPTEE III. 

 ANNULOSA AND ANNULOIDA. 



SUB-KINGDOM II. Annulosa. 



DIVISION I. Arthropoda, 



CLASS 1. Insecta. 



68. The highest class of Arthropoda is termed Insecta 

 (Lat. inseco, I cut into). It is so called on account of 

 the deep grooves which usually separate the bodies of 

 these animals into three distinct parts. 



69. Definition. Insects are distinguished from the 

 other classes of Arthropoda by the following peculiarities 

 of structure : There are three distinct sections in the 

 body head, thorax, and abdomen; the perfect animal 

 has three, pairs of legs attached to the thorax, which 

 also bears one or two pairs of wings. The abdomen is always 

 without limbs. There is one pair of antennae. Kespiratioii 

 is performed by means of air tubes or tracheae. 



70. Skeleton. As in the Annulosa generally, the hard 

 parts of insects are external. The body is covered by 

 an integument which is hardened by a horny substance, 

 termed chitine (Gr. chiton, a coat). To this exoskeleton, 

 which covers both body and limbs, the muscles are at- 

 tached. There are never more tljan twenty ringed seg- 

 ments, or somites (Gr. soma, body), in the body. Five 

 or six of these somites form the head, and three the 

 thorax. There are usually eight or nine somites in the 

 abdomen. The somites of the head are so completely 

 amalgamated as to form a single piece. The segments of 

 the thorax are also amalgamated, but not so as to be un- 

 distinguishable. The segments of the abdomen are 

 movable upon one another. 



